Double thermal resistance means that two thermal resistances form a set of thermal resistance and are installed in the outer protection tube to measure the temperature at the same point. There are dual outputs, and two thermostats can be connected. There are one or two resistor cores in a protection tube. Generally, two key temperature measurement points, temperature measurement points that are easily damaged, and temperature measurement points for interlocking control are used. This can not only ensure the accuracy of temperature measurement, but also prevent the trouble of re-disassembly after a damaged one.